Extensive tight junctions exist between epithelial cells of the epididymis and vas deferens. Normally, they provide an effective barrier to exchange between the luminal content and interstitial spaces. Following vasectomy in rodents, however, some animals lose this impermeability. More significantly, perhaps, macrophages later appear in the ductal lumen which may transport sperm across the epithelium. Since antibodies to sperm subsequently develop, studies were initiated to find a means of unequivocal identification of sperm membranes, membrane fragments, and breakdown products which may escape from the ducts. To this end, freeze-fractures of rat and guinea pig epididymal sperm were examined in detail. The studies proceeded to include changes in the particle distributions in sperm membranes after acrosomal disruption in vitro. The thrust of the work in upcoming months will be to conclude characterization of sperm membranes in situ, after acrosomal disruption, and after lysosomal incorporation. Their recognition is paramount in studying the escape of sperm membranes from the ductal lumen. Parallel studies using macromolecular tracers will also be continued.